This invention relates to a method of controlling the level of molten metal in a mold in the process of continuous casting as caused by irregular slab bulging occurring in the secondary cooling zone and by the eccentricity of the pinch rolls and other rolls, to a control apparatus thereof and to a method of continuous casting of steel.
FIG. 1 is a schematic representation of a continuous casting machine and a system of controlling the level of the molten metal in a mold, which is in conventional use.
Molten steel 1 poured into a mold 4 through a tundish 2 and a submerged entry nozzle 3 is cooled in the mold 4, whereupon a solidified shell 6 is formed. The solidification of the liquid core 7 within the solidified shell progresses and a slab 5 is formed. The slab is supported by a plurality of guide rolls 8 in the secondary cooling zone and continuously withdrawn downward by means of a plurality of pinch rolls equipped with drive motor 10.
The molten metal level in a mold is controlled in the following manner. The level of molten steel 1 is detected by a molten metal level detector 11, and a molten metal level controller 12 performs its control function according to a control logic, namely by means of proportional positions and integral motions, and drives, by using a stopper driving device 13, a stopper 14 to thereby control the rate of inflow of molten steel 1 so that the deflection from a set value of the molten metal level may become zero. In this manner, the molten metal level is maintained at a set value even when the casting condition is changed or clogging of the submerged entry nozzle 3 occurs.
FIG. 2A and FIG. 2B schematically illustrate how irregular bulging occurs. FIG. 2A shows the case of slab swelling and FIG. 2B shows the case of slab shrinking.
While, as shown in FIG. 2A, the solidified shell 6 of the slab 5 is not yet sufficiently thick, the slab 5 is apt to deform and may swell between the secondary cooling zone guide rolls 8 under the static pressure of the molten steel. When the rate of the feeding of the molten steel to the mold is constant, the molten steel level falls as shown by the arrow A. When the thickness of such a swelled slab is restored to its original thickness by secondary cooling zone guide rolls 8, the molten steel level rises.
Thus, when the slab 5 deforms easily, the portion that has once swelled is again pressed by the secondary cooling zone guide rolls 8, while the portion that is now out of contact with the secondary cooling zone guide rolls 8 swells. The molten metal level in the mold 4 will not change, if the amount of transfer of molten steel 1 in the liquid core 7 on the occasion of slab bulging is equal to the amount of molten steel 1 in the liquid core 7 on the occasion of the slab being pressed.
If, however, the casting proceeds, for some cause or other, in a manner such that the slab 5 solidifies while maintaining wavy bumps thereon, the crest-forming portions are pressed by secondary cooling zone guide rolls 8, as shown in FIG. 2B, therefore the volume of the slab shrinks, resulting in the molten metal level rising as indicated by the arrow B. This process is repeated and periodical molten metal level fluctuations occur in the mold at roll gap intervals. This state is called irregular bulging.
When this periodic change in molten metal level increases, the quality of the slab may deteriorate or a breakout may occur in some instances. Such irregular bulging may occur with a steel grade which has a high carbon content (peritectic steel) or a steel grade which has a high alloying element content. The xe2x80x9croll gapxe2x80x9d, so referred to herein, means the distance between two points in the central axis of the rolls in the direction of casting.
Meanwhile, all the roll gaps in the direction of the casting in the secondary cooling zone are generally not equal but the roll gap is smaller in a roll segment close to the mold, but the setting increases as the distant increases from the mold. Two or more segments differ in the roll gap are used in one continuous casting machine. Therefore, the above-mentioned periodical molten metal level fluctuations due to irregular bulging may contain not only one but also two or more frequency components as the case may be.
In addition, when a guide roll and/or pinch roll is eccentric, namely not straight, the slab in which the liquid core is involved is periodically subjected to a reduction and release, so that periodical molten metal level fluctuations occur in the mold. Generally, a plurality of guide rolls differing in diameter are used in one continuous casting machine and, therefore, the periodical molten metal level fluctuations due to the eccentricity of rolls may contain not only one but also two or more frequency components as the case may be.
In order to prevent the occurrence of periodical molten metal level fluctuations in the mold due to irregular bulging, a method which is disclosed in JP Kokai (Laid-open Unexamined Japanese Patent Application) H04-65742, which states that the roll gaps in the secondary cooling zone are made unequal.
FIG. 3 is a block diagram illustrating a system in the conventional use for controlling molten metal level fluctuations in continuous casting. The symbol 12 indicates a molten metal level controller, 15 a deflection calculation part calculating the difference between the set value of the molten metal level and the deflection, 16 a control logic part executing proportional position and integral motion operations, 17 the transfer function of a stopper driving device, 18 the transfer function of a stopper, 19 the transfer function of the mold, and 20 the transfer function of a molten metal level meter. In the same figure, SP is a molten metal level value (mm) as set, PV is a molten metal level value (mm) as measured by the molten metal level meter, and MV is an output (mm) of the molten metal level controller.
In JP Kokai H05-23811, there is disclosed a technique of preventing molten metal level fluctuations in the mold which comprises superimposing a sine signal for compensation on a control signal so that the molten metal level fluctuations in the mold may be eliminated, on the assumption that the periodical molten metal level fluctuations in the mold show a sine-shaped curve.
In JP Kokai H10-314911, there is disclosed a method of preventing periodical molten metal level fluctuations in the mold due to irregular bulging by providing, for advancing the phase of molten metal level deflections, a phase compensator which frequency characteristics are adjusted to the frequencies of the periodical molten metal level fluctuations in the mold, and by inputting the molten metal level deflections in this phase compensator and adding the output of this phase compensator to the operation output of the molten metal level controller, namely the control command to a sliding nozzle or stopper controller, for thereby compensating the phase delay due to the integral characteristics of the mold mass balance.
Among the technologies of suppressing the occurrence of periodical molten metal level fluctuations due to irregular bulging, the method disclosed in JP Kokai H04-65742 which comprises making unequal the roll gaps in the secondary cooling zone requires many kinds of spare roll segment, hence causes increases in cost of equipment.
When such a prior art control system as shown in FIG. 3 is used, the periodical molten metal level fluctuations may become a little greater in some instances. The reason is that the loop gain at a specific frequency of the feed back control system shown in FIG. 3 is greater than 1, so the control system becomes unstable.
According to the method of control, as disclosed in JP Kokai H05-23811, the periodical molten metal level fluctuations due to irregular bulging are assumed as one sinusoidal wave or ramp type fluctuations which increase or decrease at a constant inclination, and are assumed as fluctuations that depend on the roll gap and casting rate, hence the method cannot cope with a case in which the periodical molten metal level fluctuations include a plurality of frequency components.
According to the control method, disclosed in JP Kokai H10-314911, the frequency components of the periodical molten metal level fluctuations are input into the molten metal level controller and, therefore, the output of the molten metal level controller and the operation results of the phase compensator interfere with each other, hence the method cannot cope in the case where there are a plurality of frequencies of periodical molten metal level fluctuations.
It is an object of the present invention, which has been made in view of the above problems in the prior art, to provide a method of controlling the level of the molten metal in a mold, a control apparatus therefor and a method of continuous casting of steel which can cope with the respective frequencies and amplitudes of periodical molten metal level fluctuations due to irregular bulging and periodical molten metal level fluctuations due to the eccentricity of pinch rolls, and which, even when a plurality of frequencies of periodical molten metal level fluctuations exist, can cope with the plurality of frequencies and efficiently prevent the molten metal level fluctuations.
The gist of the present invention is as follows:
The method of control according to the invention is a method of controlling the level of the molten metal in a mold which comprises determining in advance the frequencies of periodical molten metal level fluctuations in the mold and damping selectively the predetermined frequency of frequencies of periodical molten metal level fluctuations through a notch filter installed in the control loop of the molten metal level controller. In this method of control, it is desirable that the notch filter for damping selectively the predetermined frequency and the phase compensation operation part for compensating the phase delay of the stopper opening position control signal which adjust the amount of the molten metal to be fed into the mold be involved in the control loop.
The control apparatus of the invention is a apparatus which comprises, in the control loop thereof, a molten metal level senser, an FFT analyzer, an automatic tune up device for the results from the FFT analyzer, a molten metal level controller and a notch-filter. It is desirable that this control apparatus further comprises a phase compensation calculation part consisting of a band pass filter, a phase compensator and a phase compensation gain calculation part.
The method of continuous casting of steel, according to the invention, is a method of casting a molten metal into slabs which are rectangular in shape, using the control method and the control apparatus mentioned above.